If you're going earlier, the comments about it being a rail line grow. One should also keep in mind that there already is both a
railroad in Alaska running from the ports of Seward and Whittier to Anchorage and then well inland to Fairbanks, as well as
another that was built from Skagway to Whitehorse. The further north line on the Canadian network is Fort Nelson, British Columbia, which is part of the former Pacific Great Eastern Railway (which became the British Columbia Railway in 1972), which was opened to Fort Nelson in 1971 though the most likely path to get there had been known for many years.
The best way might be for Alaska to have its minerals discovered sooner as well as a breakdown in relations between the USSR and US which spurs a fear in both Canada and the US to make sure they can get troops to Alaska if needed. That gets figured in the 1920s, and the Alaska Railway begins to form during the 1920s. During the Great Depression funds become scarce, but both Washington and Ottawa provide the funds for the Alaska Railway to go southeast from Fairbanks while the Pacific Great Eastern to finish first its line to Prince George in the mid-1930s, then continue north. The Alaska Railway reaches the Yukon in 1937 while the PGE gets to Fort St. John in 1939, just in time for war in Europe to break out.
After Pearl Harbor the resources of Alaska become important, and fear of Japanese submarines gets the United States to quickly get involved. The United States takes over the White Pass and Yukon Route in 1942 and rapidly expands the Alaska Railway to meet the WP&YR in Whitehorse in 1944, while also providing the funds to the PGE to go north to Fort Nelson. The railroad is not completed during the war, but it doesn't take long after for the job to be finished. The Fort Nelson-Whitehorse section of the line is finished in the early summer of 1948 and enters operation in 1949, with the line almost immediately being used as a conduit to take loads to Alaska. WP&YR is integrated into the Alaska Railroad in 1952 and is rebuilt as a standard-gauge railroad between 1952 and 1958, and almost immediately the Alaska Railroad and Pacific Great Eastern are busy roads. The heavy coal and mineral traffic on several routes results in near-constant improvements on the line - the line is electrified from Whitehorse to Skagway in 1957 and Fort Nelson in 1959, and the anticipation of huge hydroelectric power from the Rampart Dam project results in the railroad electrifying its entire mainline from Whitehorse to Fairbanks between 1960 and 1964. The railroad over this period shifts from being a smaller line to being a heavy-hauler route.
The intertwined PGE and ARR had long been considered for a merger, and financial problems for the PGE and the ARR's huge debts result in both seeking reorganization in the early 1970s....but after the collapse of the behemoth Penn Central Railroad in 1970, America's bankrupt railroads are soon being reorganized on a mammoth scale, and with Canadian National Railways becoming involved in the operation of the PGE in 1971 and the inclusion of the bankrupt Chicago, Milwaukee, St. Paul and Pacific into the USRA-created Consolidated Rail Corporation, the decision was made to do the same with the ARR and PGE, with the British Columbia Government getting an ownership stake in Conrail as a result. The move also resulted in the building of a new mainline from Metaline Falls, WA, to Lillooet, BC, by way of Trail, Kelowna and Kamloops, BC, as well as a new line from North Vancouver, BC to Seattle, WA, most of it a former Northern Pacific mainline surplus as a result of the Burlington Northern merger.
Conrail made its Pacific Coast division a symbol of what exactly to do for American heavy-hauler railroading. All of the British Columbia mainline divisions were electrified through the 1970s and 1980s, as well as Conrail's Pacific Extension mainline from Seattle and Tacoma all the way to McLaughlin and Rapid City, South Dakota, as well as building the Powder River divisions as electrified from the start. Conrail's developments and crowding at Puget Sound ports all but eliminated the movement of minerals, energy or materials from Alaska by ship, with the Alaska and British Columbia divisions of Conrail handling nearly all of the loads headed to Canada or the Lower 48. The line's creation also created huge quantities of forestry traffic, and the building of the Alaska Pipeline and huge mines in both British Columbia and Alaska also resulted in huge mineral traffic. By the 1980s, Conrail's fast delivery times to the rest of North America spawned an industry exporting seafood caught in the rich waters around Alaska, and growing production of natural gas saw large quantities of natural gas being delivered to customers using tank trains. The railroad's infrastructure - 175-pound rail, concrete sleepers, dense roadbed, sturdy bridges and tunnels, electrification and use of abundant local hydroelectric power and centralized traffic control - resulted in the Alaska Railroad having some of the heaviest trains anywhere in the world, and Conrail's 'Mercury' piggyback service along the route and the development of better roads to many communities resulted in marked drops in the cost of importing goods to the region. By the end of the century, the railroad was one of the most important pieces of infrastructure anywhere in North America, and Conrail, thanks to both its huge efforts and continual expansion of its network and market over routes that would otherwise be abandoned by other railroads, was an incredibly profitable enterprise. Two separate attempts by Washington to privatize Conrail in the 1990s and 2000s were both vetoed, and Conrail's moves were copied heavily by the behemoth Canadian National Railways in the 1970s, 1980s and 1990s.